The conventional portable computing device, such as a personal data assistant (PDA) or wireless telephone, has a liquid crystal display (LCD), a LCD backlight, and a keyboard/keypad. Typically, the display backlight is enabled when the device is powered on or when a key on the keyboard/keypad is depressed. The intensity of light produced by the backlight is factory set to a fixed level that allows the information displayed on the LCD to be viewable in moderate ambient light levels. However, the backlight tends to be excessively bright at dim ambient light levels, and insufficiently bright at very bright ambient light levels. In both of these situations, the clarity of the information displayed on the LCD is comprised. Accordingly, attempts have been made to make the information displayed on a LCD more readily viewable over a wider range of ambient light levels.
For instance, Keiji (Patent Abstracts of Japan, Pub. No. 11-260572) describes a lighting system that changes the illumination level of the LCD, based on the intensity of ambient light detected. In one embodiment, the lighting system comprises a liquid crystal display having a Light Emitting Diode (LED) backlight and a light guide; a light sensor disposed at an end face of the light guide; and control means for varying the intensity of light emitted by backlight. The light sensor measures the luminous energy at two or more wavelengths. The control means determines the energy of light measured by the light sensor, while momentarily setting the radiant output energy of the backlight to zero. The control means then sets the radiant output energy of the backlight based on the energy of light so measured.
Since the light sensor is disposed within the LCD, the light sensor is not prone to erroneous intensity measurements resulting from dirt contaminating the optical surface of the sensor. However, incorporating the light sensor into the LCD increases the manufacturing cost of the LCD. Therefore, there remains a need for an improved mechanism for rendering the information displayed on a LCD more readily viewable over a wide range of ambient light levels.